1. Field of the Invention
This present invention relates to tracking of assets, such as railcars and their cargos, using the Global Positioning System (GPS) and, more particularly, to a method and apparatus for reducing data transmitted between railcars of a train without going through a transponder.
2. Description of the Prior Art
Goods shipped from a manufacturing plant, warehouse or port of entry to a destination are normally required to be tracked to assure their timely and safe delivery. Tracking has heretofore been accomplished in part by use of shipping documents and negotiable instruments, some of which travel with the goods and others of which are transmitted by post or courier to a receiving destination. This paper tracking provides a record which is completed only on the safe delivery and acceptance of the goods. However, there sometimes is a need to know the location of the goods while in transit. Knowledge of the location of the goods can be used for inventory control, scheduling and monitoring.
Shippers have provided information on the location of goods by tracking their vehicles, knowing what goods are loaded on those vehicles. Goods are often loaded aboard shipping containers or container trucks, for example, which are in turn loaded aboard railcars. Various devices have been used to track such railcars. For example, passive radio frequency (RF) transponders mounted on the cars have been used to interrogate each car as it passes a way station and supply each car's identification (ID). This information is then transmitted by a radiated signal or land line to a central station which tracks the locations of the cars. This technique, however, is deficient in that any railcar on a siding cannot pass a way station until it has left the siding. Moreover, way station installations are is expensive, requiring a compromise that results in way stations being installed at varying distances, depending on the track layout. Thus, the precision of location information varies from place to place on the railroad.
Recently, mobile tracking units have been used for tracking various types of vehicles, such as trains. Communication has been provided by means of cellular mobile telephone or RF radio link. Such mobile tracking units are generally installed aboard the locomotive which provides a ready source of power. However, in the case of shipping containers, container truck trailers and railcars, a similar source of power is not readily available. Mobile tracking units which might be attached to containers and vehicles must be power efficient in order to provide reliable and economical operation. Typically, a mobile tracking unit includes a navigation set, such as a Global Positioning System (GPS) receiver or other suitable navigation set, responsive to navigation signals transmitted by a set of navigation stations which may be either space-based or earth-based. In each case, the navigation set is capable of providing data indicative of the vehicle location based on the navigation signals. In addition, the mobile tracking unit may include a suitable electromagnetic emitter for transmitting to a remote location the vehicle location data and other data acquired from sensing elements on board the vehicle. Current methods of asset localization require that each item tracked be individually equipped with hardware which determines and reports location to a central station. In this way, a tracked asset is completely "ignorant" of other assets being shipped or their possible relation to itself. In reporting to the central station, such system requires a time-bandwidth product which scales approximately with the number of assets being reported. The aggregate power consumption over an entire such system also scales with the number of assets tracked. Further, since both the navigation set and the emitter are devices which, when energized, generally require a large portion of the overall electrical power consumed by the mobile tracking unit, it is desirable to control the respective rates at which such devices are respectively activated and limit their respective duty cycles so as to minimize the overall power consumption of the mobile tracking unit.
In order for a network of mobile tracking units to operate reliably, it is desirable to know the distance between a "master" tracking unit and each of the remaining tracking units in the network, which may be considered to be "slave" tracking units, and also whether a slave tracking unit is ahead of or behind the master tracking unit; that is, whether the slave tracking unit is closer to, or farther away from, the front locomotive in the direction of train movement. There is also a need to determine the individual communication path losses from the master tracking unit to individual slave tracking units and to determine the quality of the channel as to its capacity for passing data between the master and the slave tracking units. The function of master tracking unit is assigned to a specific tracking unit according to a protocol within the network.